Alkaline electrolyser (AE) systems still offer the largest system size at the lowest cost but are unsuited to some forms of renewable generation, according to a study of the costs of various different electrolyser technologies by the Oxford Institute for Energy Studies (OIES).

The paper divides the most promising electrolyser technologies into six major groups: AE; acidic, which includes proton-exchange-membrane (PEM) technologies; solid oxide electrolysers (SOEs); acidic-alkaline amphoteric electrolysers, which include anion-exchange-membrane (AEM) technologies; microbial; and photo-electrochemical.

As the paper notes, each technology has its strengths and drawbacks depending on the application and operating environment.

“Electrolysers’ performance features represent a set of trade-offs among multiple factors such as cost, quantity of noble metals used, technology readiness and the ability to operate in a flexible mode,” says the paper, titled Cost-competitive green hydrogen: how to lower the cost of electrolysers?

“Electrolysers’ performance features represent a set of trade-offs among multiple factors” OIES

Currently only AE and PEM systems are fully commercialised. AE systems have the advantage of using low-cost components for both electrodes, as well as long operating lifetimes.

The investment costs for AE are usually between $800/kW and $1,500/kW, which is around 2-2.5 times lower than for PEM electrolysers—the next commercially available generation technology—the study finds.

But AE systems also have a number of significant drawbacks, including their high lower limit of minimum load, which prevents their coupling with some types of renewables.

“Although this drawback can be partially mitigated through the use of batteries and control systems, AEs do not seem to be the best option for balancing variable renewable energy sources such as wind and solar PV,” says the report.

PEM pal

Instead, PEM facilities are the most promising option for pairing hydrogen production with solar and wind generation, the study finds.

The low level of partial load required by the electrolysers and their ability to rapid cycle makes them suitable for running on intermittent power sources.

However, one of the major barriers impeding the use of PEM systems more widely is the high cost of catalysts and electrodes they use, especially platinum. As a result, a number of manufacturers are looking at ways to reduce the use of—or recycle—these materials. 

AEM systems can avoid this problem by using inexpensive catalysts and core materials for electrodes. But they suffer from the same minimum load issues as AE systems and have more limited operating lifetimes than other technologies.

Solid as a rock

Alongside AE and PEM hydrogen production technologies, SOEs are widely expected to become the third main water electrolysis technology in the foreseeable future.

SOEs have an efficiency of 80-90pc, which is higher than any of the technology’s competitors. They also have a low level of partial load and can be run at low operating costs with inexpensive catalysts.

The high temperatures that improve the technology’s efficiency also have drawbacks, however, as they result in longer startup and break-in times.

“This may potentially hamper the system’s coupling with intermittent renewables, despite the relatively low minimum load,” says the study.

High temperatures also affect the longevity and durability of SOEs.

“These features of SOEs leave some space for improvement before they can become feasible rivals for alkaline and PEM facilities,” says the report.

Falling cost curve

Electrolysers are expected to benefit from module efficiency driven by R&D and economies of scale in the same way that renewables did, the study finds. SOEs, in particular, offer good potential for cost reductions.

Government support policies—including direct financial support of pilot projects—are vital in enabling these cost reductions, according to the paper.

“Supportive policies should focus not only on the promotion of renewable energy and electrolysers, but also on the build-up of hydrogen infrastructure as well as demand among end-users, industries and sectors,” it says.

---

Author: Tom Young